Cast Iron Material Testing

Cast iron material inspection mainly conducts scientific testing and analysis on the chemical composition analysis, metallographic structure examination, mechanical property testing, non-destructive testing, dimension and geometric tolerance testing and special property testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Chemical composition analysis: Aiming to precisely determine the contents of the five routine elements carbon, silicon, manganese, sulfur and phosphorus in the cast iron and the alloying elements such as the chromium, nickel, molybdenum and copper. This is the basis of judging the cast iron designations and predicting their mechanical properties and process properties. The common methods include the spark direct-reading spectrometry, carbon-sulfur analyser method and wet chemical analysis, to ensure that the material compositions meet the requirements of the standards such as GB/T 9439 and ASTM A48.

Metallographic structure examination: Observing the micro-structures of the cast iron through the optical microscopes or scanning electron microscopes is the key to evaluating its internal quality. The core observation contents include the graphite morphologies (flake, spheroidal and vermicular), sizes and distributions, as well as the types and proportions of the metal matrices (ferrite, pearlite and cementite). This testing is directly related to the properties of the material such as the strength, hardness, toughness and wear resistance.

Mechanical property testing: Mainly evaluates the behavior of the cast iron under the applied forces. The core items include the tensile strength, yield strength, elongation, hardness and impact toughness. The testing is carried out according to the standards such as GB/T 228, GB/T 231 and ASTM E8, using the equipment such as the universal material testing machines, Brinell/Rockwell/Vickers hardness testers and impact testing machines, providing the direct data support for the part design material selection and safety evaluation.

Non-destructive testing: Under the premise of not destroying the castings, their surface and internal defects are probed. The common methods include the ultrasonic testing (UT) for finding the internal shrinkage cavities and cracks; the radiographic testing (RT) for displaying the internal three-dimensional defect morphologies; the magnetic particle testing (MT) and penetrant testing (PT) are mainly used to detect the open defects such as the surface cracks, cold shuts and blowholes of the castings, ensuring the structural integrity of the castings.

Dimension and geometric tolerance testing: The tools such as the coordinate measuring machines (CMM), laser scanners, dedicated gauges and callipers are used for the precise measurement of the key dimensions and geometric tolerances (such as the flatness, roundness and coaxiality) of the castings. This testing ensures that the castings meet the drawing design requirements and satisfy the precision needs of the subsequent machining and assembly, and is the important link of the quality control.

Special property testing: According to the specific application scenarios of the cast iron, the special property evaluations may need to be carried out. For example, the heat-resistant cast iron needs the high-temperature oxidation resistance and growth resistance testing; the corrosion-resistant cast iron needs the salt spray testing or chemical medium immersion testing; the wear-resistant cast iron needs the wear testing. These tests verify the applicability and reliability of the material under the specific working conditions.

Testing Scope

Grey iron castings: The testing emphases are the flake graphite morphologies, length distributions and matrix structures, and the mechanical properties take the tensile strength and hardness as the core indicators. Suitable for the parts mainly bearing the compressive stresses such as the engine blocks, machine tool beds and transmission housings, and the casting defects (such as the shrinkage porosity and sand holes) and machinability need to be strictly monitored.

Ductile iron castings: The core testing items are the graphite nodularity, nodularity grades, nodule sizes and matrix structures (ferrite/pearlite proportions). Their mechanical properties are close to the steel, and the tensile strength, elongation and impact toughness need to be emphatically tested. Widely applied to the key parts with the high strength and toughness requirements such as the crankshafts, gears and bridge bearings.

Compacted graphite iron castings: The testing key is confirming the vermicular graphite morphologies and their proportions. In terms of the properties, they have both the thermal conductivity and thermal shock resistance of the grey iron and the high strengths of the ductile iron. The special testing of their thermal fatigue properties, high-temperature strengths and thermal conductivity needs to be carried out, often used for the thermal cyclic load working conditions such as the automobile exhaust pipes, ingot molds and brake drums.

Malleable iron castings: The temper graphite morphologies after the annealing and the decarburized layer depths need to be tested. In terms of the properties, they are characterized by the higher toughness, plasticity and good machinability. The testing scope includes the small castings with the complex shapes and needing to bear the impacts and vibrations such as the hardware tools, pipe fittings and agricultural machinery parts.

Alloy iron castings: On the basis of the routine testing, the alloying element content analysis and the evaluation of their effects on the structures and properties must be added. For example, the high-chromium iron needs the carbide types, morphologies and hardnesses testing; the silicon-molybdenum ductile iron needs the high-temperature lasting strength evaluation. Suitable for the special working conditions such as the wear-resistant liners, rolls and heat-resistant stoves.

Cast iron raw materials and melts: The testing scope extends to the compositions of the furnace charges such as the pig iron, scrap steel and ferroalloys, as well as the fast furnace-front analysis before the melt tapping (such as the thermal analysers measuring the carbon equivalent and the oxygen probes measuring the oxygen activities). This is the source of controlling the final casting quality, aiming to optimize the melting processes and reduce the fluctuations.

Testing Methods

Spectroscopic analysis method: Mainly including the spark discharge atomic emission spectrometry (OES) and X-ray fluorescence spectrometry (XRF). The OES can quickly and simultaneously determine the contents of the multiple elements in the cast iron, with the high precision, and is the mainstream method of the furnace-front control and finished product inspection. The XRF is suitable for the scenarios with the sample destruction requirements or the analysis of the light elements (such as C and S), but the precision is slightly lower than the OES.

Metallographic microscopic analysis method: According to the standards such as GB/T 9441 and ASTM A247, the specimens after the grinding, polishing and etching are observed and rated under the optical microscopes. The parameters such as the graphite area rates, nodularity and pearlite contents can be quantified through the image analysis software. This method is intuitive and reliable, and is the fundamental means of studying the structure-property relationships of the cast iron.

Tensile testing method: According to the GB/T 228.1 standard, the specimens machined into the standard proportions are pulled on the universal testing machines at the constant rates until the fracture, recording the force-displacement curves, thereby calculating the key mechanical parameters such as the tensile strength, yield strength and elongation after fracture. For the grey iron with the greater brittleness, usually only the tensile strength is determined.

Brinell hardness testing method: According to the GB/T 231.1 standard, the cemented carbide ball indenters of the larger diameters are adopted, the specified test forces are applied and pressed into the specimen surfaces, and after maintaining for a certain time, the indentation diameters are measured and converted into the Brinell hardness values (HBW). This method has the large indentation areas and can better reflect the average hardnesses of the cast iron structures, especially suitable for the materials with the uneven structures such as the grey iron.

Ultrasonic flaw detection method: Using the principle that the high-frequency sound waves reflect when they encounter the defect interfaces during the propagation in the castings. Through the probes receiving the reflected waves, the positions and equivalent sizes of the defects are displayed on the instrument screens. This method is sensitive to the planar defects such as the internal shrinkage cavities and cracks, has the strong penetration, and is suitable for the internal quality general surveys of the large and medium thick-wall castings.

Three-dimensional coordinate measurement method: The probes of the coordinate measuring machines (CMM) contact or the lasers scan the casting surfaces, obtaining the spatial coordinates of the large numbers of points, comparing with the CAD models through the software, and precisely evaluating the dimension errors and geometric tolerances. This method has the high automation degree and good repeatability, suitable for the full-dimension testing of the complex curved surfaces and precision castings.

Testing Standards

Domestic Standards:

1. GB/T 9439, Grey iron castings

2. GB/T 9441, Metallographic test for spheroidal graphite cast iron

3. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

4. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

5. GB/T 231.1, Metallic materials — Brinell hardness test — Part 1: Test method

6. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

International Standards:

1. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

2. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

3. ASTM A247, Standard test method for evaluating the microstructure of graphite in iron castings

4. ASTM E8, Standard test methods for tension testing of metallic materials

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Direct-reading spectrometer: The core fast chemical composition analysis equipment. Through the arc or spark exciting the samples to generate the characteristic spectra, after the grating dispersion and photomultiplier tube detection, the contents of more than ten elements in the cast iron can be simultaneously determined within tens of seconds. The instrument needs to be regularly calibrated with the standard samples to ensure the analysis precision, and is the indispensable equipment of the casting site quality control.

Metallographic microscope and image analysis system: Composed of the optical microscopes, high-resolution CCD cameras, computers and professional image analysis software. Not only used for the manual observation and rating, but also can automatically identify, segment and count the graphite particles and matrix structures, outputting the quantitative data reports, greatly improving the objectivity and efficiency of the metallographic examination.

Universal material testing machine: Used to complete the mechanical testing such as the tensile, compression and bending. The modern testing machines generally adopt the servo motors or hydraulic drives, equipped with the high-precision load transducers and extensometers, controlled by the computers and automatically collecting the data, drawing the curves and calculating the results. It is the benchmark equipment of obtaining the strength and plasticity indicators of the material.

Hardness tester series: According to the different principles and application ranges, mainly including the Brinell hardness testers (measuring HBW, suitable for the coarse-grained materials), Rockwell hardness testers (measuring HRC/HRB, fast and convenient), Vickers hardness testers (measuring HV, suitable for the thin layers or tiny areas) and Leeb hardness testers (portable, used for the on-site large workpiece testing). The appropriate hardness testers need to be selected according to the standards.

Non-destructive testing equipment group: Including the ultrasonic flaw detectors (with the probes of the different frequencies and angles), X/γ-ray flaw detection machines (with the films or digital imaging plates), magnetic particle flaw detectors (including the wet, dry and fluorescent magnetic particles) and penetrant flaw detection reagent kits. These equipment work together to realize the all-directional detection of the castings from the surfaces to the internal defects.

Coordinate measuring machine: The high-precision geometric dimension measurement equipment, composed of the rigid structures, precision guide rails, detection systems, control systems and measurement software. Its measurement precision can reach the micron level, and it can carry out the complex dimension, shape and position tolerance evaluations. It is the final arbitration tool of verifying whether the castings meet the design drawings.